Memory voltage cycle adjustment

ABSTRACT

The present disclosure includes various method, device, system, and module embodiments for memory cycle voltage adjustment. One such method embodiment includes counting a number of process cycles performed on a first memory block in a memory device. This method embodiment also includes adjusting at least one program voltage, from an initial program voltage to an adjusted voltage, in response to the counted number of process cycles.

PRIORITY INFORMATION

This application is a Divisional of U.S. patent application Ser. No.12/754,919 filed Apr. 6, 2010, which is a Continuation of U.S. patentapplication Ser. No. 12/354,975 filed Jan. 16, 2009 now U.S. Pat. No.7,715,239 issued May 11, 2010, which is a Divisional of U.S. patentapplication Ser. No. 11/414,966 filed May 1, 2006 now U.S. Pat. No.7,495,966 issued Feb. 24, 2009 the specification of which isincorporated by reference herein.

TECHNICAL FIELD

The present disclosure relates generally to semiconductor devices and,more particularly, to memory devices having memory voltage cycleadjustment

BACKGROUND

Memory devices are typically provided as internal, semiconductor,integrated circuits in computers or other electronic devices. There aremany different types of memory including random-access memory (RAM),read only memory (ROM), dynamic random access memory (DRAM), synchronousdynamic random access memory (SDRAM), and flash memory, among others.

Flash memory devices are utilized as non-volatile memory for a widerange of electronic applications. Flash memory devices typically use aone-transistor memory cell that allows for high memory densities, highreliability, and low power consumption.

Uses for flash memory include memory for personal computers, personaldigital assistants (PDAs), digital cameras, and cellular telephones.Program code and system data, such as a basic input/output system(BIOS), are typically stored in flash memory devices. This informationcan be used in personal computer systems, among others.

Two common types of flash memory array architectures are the “NAND” and“NOR” architectures, so called for the logical form in which the basicmemory cell configuration of each is arranged. In the NOR arrayarchitecture, the floating gate memory cells of the memory array aretypically arranged in a matrix.

The gates of each floating gate memory cell of the array matrix aretypically coupled by rows to word select lines and their drains arecoupled to column bit lines. The NOR architecture floating gate memoryarray is accessed by a row decoder activating a row of floating gatememory cells by selecting the word select line coupled to their gates.The row of selected memory cells then place their data values on thecolumn bit lines by flowing different currents depending on if aparticular cell is in a programmed state or an erased state.

A NAND array architecture also arranges its array of floating gatememory cells in a matrix such that the gates of each floating gatememory cell of the array are coupled by rows to word select lines.However each memory cell is not directly coupled to a column bit line byits drain. Instead, the memory cells of the array are coupled togetherin series, source to drain, between a source line and a column bit line.

The NAND architecture memory array is accessed by a row decoderactivating a row of memory cells by selecting the word select linecoupled to their gates. A high bias voltage is applied to a select gatedrain line SG(D).

In addition, the word lines coupled to the gates of the unselectedmemory cells of each group are driven (e.g., at Vpass) to operate theunselected memory cells of each group as pass transistors so that theypass current in a manner that is unrestricted by their stored datavalues. Current then flows from the source line to the column bit linethrough each series coupled group, restricted only by the selectedmemory cells of each group. This places the current encoded data valuesof the row of selected memory cells on the column bit lines.

In some memory cells, the memory performance, e.g., programming speed,may increase as the number of program/erase cycles increases. However,this condition may make the affected cells more susceptible toover-programming. For instance, when a voltage is applied to the cell,the conditioning of the cell may cause the cell to be over charged andthereby cause an incorrect result when read and/or verified.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic of a portion of a NAND memory array that can beused with embodiments of the present disclosure.

FIG. 2A illustrates a pulse technique for incrementally programmingstorage elements of a memory cell array before a number of cycles haveoccurred according to an embodiment of the present disclosure.

FIG. 2B illustrates a pulse technique for incrementally programmingstorage elements of a memory cell array after a number of cycles haveoccurred according to an embodiment of the present disclosure.

FIG. 3 illustrates a periodic decrease in the Vpgm voltage as the numberof cycles increases for an embodiment of the present disclosure.

FIG. 4A illustrates a distribution of voltage thresholds of a group ofnon-volatile memory cells that have been individually programmed intoone of four states before a number of cycles have occurred according toan embodiment of the present disclosure.

FIG. 4B illustrates a distribution of voltage thresholds of a group ofnon-volatile memory cells that have been individually programmed intoone of four states after a number of cycles have occurred according toan embodiment of the present disclosure.

FIG. 5 illustrates verify and read voltage difference changes for anumber of memory cells before and after cycling for an embodiment of thepresent disclosure.

FIG. 6A illustrates verify and read voltages for a number of memorycells as the number of cycles increases according to an embodiment ofthe present disclosure.

FIG. 6B illustrates verify and read voltages for a number of memorycells as the number of cycles increases according to an embodiment ofthe present disclosure.

FIG. 7 is a functional block diagram of an electronic system having atleast one memory device in accordance with an embodiment of the presentdisclosure.

FIG. 8 is a functional block diagram of a memory module having at leastone memory device in accordance with an embodiment of the presentdisclosure.

DETAILED DESCRIPTION

In the following detailed description of the present disclosure,reference is made to the accompanying drawings that form a part hereof,and in which is shown by way of illustration how various embodiments ofthe disclosure may be practiced. These embodiments are described insufficient detail to enable those of ordinary skill in the art topractice the embodiments of this disclosure, and it is to be understoodthat other embodiments may be utilized and that process, electrical, ormechanical changes may be made without departing from the scope of thepresent disclosure.

The terms wafer and substrate used herein include any base semiconductorstructure. Both are to be understood as including silicon-on-sapphire(SOS) technology, silicon-on-insulator (SOI) technology, thin filmtransistor (TFT) technology, doped and undoped semiconductors, epitaxiallayers of silicon supported by a base semiconductor, as well as othersemiconductor structures well known to one of ordinary skill in the art.Furthermore, when reference is made to a wafer or substrate in thefollowing description, previous process steps may have been utilized toform regions/junctions in the base semiconductor structure. Thefollowing detailed description is, therefore, not to be taken in alimiting sense, and the scope of the present disclosure is defined onlyby the appended claims and equivalents thereof.

FIG. 1 illustrates a portion of a NAND flash memory array the can beused with various embodiments of the present disclosure. However, theembodiments of the present disclosure are not limited to use with such amemory array.

As shown in FIG. 1, the memory array 100 includes a number of word lines106 and intersecting bit lines 108. For ease of addressing in thedigital environment, the number of word lines and the number of bitlines are typically each some power of two (e.g., 256 word lines 106 by4,096 bit lines 108).

In various devices and systems, memory array 100 can be divided intodiscrete blocks of cells that can be erased together. Such memory blocksmay be referred to as an erase unit.

In the example in FIG. 1, the figure illustrates that the memory array100 includes a number of cells that are being programmed 101, 102, and103. Additionally, in the illustrated embodiment of FIG. 1, the array100 also includes a number of Vpass mode program disturb cells 110, 111,112, 113, 114, 115, 116, 117, and 118 and a number of Vpgm mode programdisturb cells 120 and 121.

These non-volatile memory cells of each NAND string are connected inseries source to drain between a source select gate (i.e., SGS), whichcan, for example, be a field-effect transistor (FET), and a drain selectgate (e.g., SOD), which can, for example, be a FET.

During a program operation to program a number of memory cells (e.g.,memory cells 101, 102, or 103), the selected word line 105, coupled tothat cell or cells 101 to 103, may be supplied by a number ofprogramming pulses. As discussed further in connection with FIGS. 2A and2B below, in many memory arrays, these voltages can start at a voltageof around 16V and may incrementally increase, such as in a steppedfashion, to around 20V.

The bit line, coupled to cells 101 to 103, may be brought to groundpotential, which can provide a gate to source potential of 20V acrossthe cells 101 to 103 that are to be programmed. The unselected wordlines (e.g., the word lines without cells to be programmed) may bebiased at a pass voltage Vpass that can be around 9-10V.

The unselected cells (e.g., cells 120 and 121) on the selected word line105 can also have the 20V programming pulse applied. In order to inhibitthese cells (e.g., 120 and 121) from being programmed, their bit lines108 may be biased to an inhibiting potential (e.g., a supply potentialVcc that can be about 3-6V, for example).

As NAND flash memory is scaled, parasitic capacitance coupling between aselected word line and adjacent word lines can become problematic. Theparasitic coupling can cause neighboring cells to become prone to havingtheir threshold voltages raised, which can result in an unprogrammedcell appearing to be programmed, for example. One type of parasiticcapacitance coupling is referred to as a program disturb condition. In aprogram disturb condition, a programming operation for one page inducesa change in bit value in another, unrelated page.

A program disturb condition typically appears in during certainoperation modes. For example, in a boosting mode, the cell's channeltypically has at a positive boosting voltage (e.g., 6V) with respect tothe gate and the gate typically has a voltage at Vpgm (e.g., 20V).During the Vpass mode, the cell's channel is typically at ground and thegate is typically at Vpass (e.g., 10V). In either of this modes, aprogram disturb can arise.

In FIG. 1, the cells 120, 121 on the selected word line 105 andinhibited bit lines are influenced by boosting mode program disturb. Theneighboring cells 110-118 that are coupled to the enabled bit linesexperience Vpass mode program disturb.

In a positive way, program disturb is often degraded as the number ofprocessing cycles increases. As used herein, a processing cycle canrefer to a program (e.g., a write) operation and/or an erase operationthat can be performed on a memory block of a memory array (e.g., array100 shown in the embodiment of FIG. 1).

In various devices and systems, program/erase operations can beperformed on memory blocks on a block by block basis, and the number ofprogram/erase cycles performed on a memory block can be stored withinmemory array 100. The quantity of program/erase cycles performed on amemory block can be referred to as an “experience count,” or “hotcount.” The hot count of a memory array, and/or of each memory blockwithin the memory array, can be monitored by a system controller (e.g.,control circuitry 712 of the embodiment shown in FIG. 7).

As the quantity of program/erase cycles increases, the voltagedifference between the programmed state and the erased state narrows.This factor may help increase the performance of the memory array.

However, this factor also makes the affected cells more susceptible toover-programming as the threshold voltage, Vt, narrows. This is a resultof the program disturb causing an increasing threshold voltage as thequantity of program/erase cycles increase.

FIG. 2A illustrates a pulse technique for incrementally programmingstorage elements of a memory cell array before a number of cycles haveoccurred according to an embodiment of the present disclosure. The pulsetechnique shown in FIGS. 2A and 2B may be used with single level memorycell arrays (SLCs) and/or multiple level memory cell arrays (MLCs).

In the embodiment described by the illustration in FIG. 2A, a memorycell's programming process includes four voltage pulses (e.g., 16.0V,16.6V, 17.2V, and 17.8V). However, embodiments of the present disclosureare neither limited to four pulses nor to a particular starting orending pulse voltage. For example, the number of pulses and/or thestarting and ending pulse voltage before a number of cycles (e.g.,before a number of program/erase cycles) can depend on various factorsincluding the type of memory cell array.

As one of ordinary skill will appreciate, and as discussed furtherbelow, verify pulses can be utilized between the program pulses depictedin FIG. 2A. Verify pulses can, for example, be positioned between the16.0V and 16.6V pulses, etc.

The number of verify pulses between program pulses can depend on thenumber of states represented by the memory cell. For example, a programvoltage signal used to program a four state memory cell can have threeverify pulses between the program, pulses which increase incrementally.For instance, a first verify pulse may be at 0.5V, a second verify pulsemay be at 1.5V, and a third verify pulse may be at 3.0V.

FIG. 2A also depicts the voltage difference between pulses (“step upvoltage”) 210-1, 210-2, and 210-3 as being equal values of 0.6V.Embodiments of the present disclosure are not so limited (e.g., step upvoltages 210-1, 210-2, and 210-3 can be different values from each otherand/or greater or less than 0.6V).

FIG. 2B illustrates a pulse technique for incrementally programmingstorage elements of a memory cell array after a number of cycles haveoccurred according to an embodiment of the present disclosure.

FIG. 2B includes four voltage pulses (e.g., 15.4V, 15.9V, 16.4V, and16.9V). However, embodiments of the present disclosure are neitherlimited to four pulses nor to a particular starting or ending pulsevoltage. For example, the number of pulses and/or the starting andending pulse voltage after a number of processing cycles (e.g.,program/erase cycles) can depend on various factors including the typeof memory cell array. As one of ordinary skill will appreciate, and asdiscussed further below, verify pulses can also be utilized between theprogram pulses depicted in FIG. 2B (e.g., between the 15.4V and 15.9Vpulse, etc).

FIG. 2B also depicts the voltage difference between pulses (“step upvoltage”) 220-1, 220-2, and 220-3 as being equal values of 0.5V.Embodiments of the present disclosure are not so limited, e.g., step upvoltages 220-1, 220-2, and 220-3 can be different values from each otherand/or greater or less than 0.5V.

In various embodiments of the present disclosure, the programmingvoltage pulses (e.g., programming voltage signal) shown in FIG. 2A canbe applied to a wordline in a memory block in order to program memorycells until a number of processing cycles have occurred (e.g., until1,000, 10,000, 50,000, 100,000, etc. program/erase cycles haveoccurred). Thereafter, according to various embodiments, adjustedprogramming voltage pulses as shown in FIG. 2B can be applied towordlines in the memory block, for example, in order to program thememory cells.

For instance, the programming voltage signal of the embodimentrepresented in FIG. 2A can be applied before the quantity of processingcycles reaches a count of 10,000, and then the decreased programmingvoltage signals of FIG. 2B can be applied thereafter.

Embodiments of the present disclosure are not limited to the aboveexample shown in FIGS. 2A and 2B. For instance, in various embodiments,an adjusted programming voltage signal (e.g., a signal in which thevoltage pulses and/or step up voltages are decreased) may be appliedafter more or fewer program/erase cycles have occurred.

The embodiment shown in FIGS. 2A and 2B illustrates a decrease inprogramming voltage pulses of 0.6V-0.9V between the before cycling andafter cycling programming voltage signals. In various embodiments, thedecrease can be the same voltage for two or more of the programmingvoltage pulses (e.g., a 0.6V decrease for all programming voltage pulsesor e.g., 0.6V for a first pulse, 0.7V for a second pulse, and 0.8V forthird and fourth pulses).

In some embodiments, such as the embodiment illustrated in FIGS. 2A and2B, the step up voltage can also be decreased after cycling (e.g., thestep up voltage goes from 0.6V before cycling to 0.5V after cycling).However, in some embodiments, only the programming voltage pulses or thestep up voltage may be decreased after cycling. For example, the initialprogramming voltage or pulses may be decreased after a number ofprogramming cycles while the step up voltage remains constant. Forinstance, in some embodiments the step voltage before a number ofprogram/erase cycles may remain at a value of 0.5V after a number ofprogram/erase cycles, e.g., 1,000 cycles, have occurred.

Furthermore, it is possible to adjust the programming voltage signalmore than one time. That is, the programming voltage signal asillustrated in FIG. 2A, for example, can be applied to wordlines in amemory block in order to program memory cells until the hot countreaches a predetermined quantity such as 1,000 counts.

In such embodiments, an adjusted programming signal (e.g., the adjustedsignal illustrated in FIG. 2B) can then be applied to program memorycells in a memory block until the hot count reaches anotherpredetermined quantity such as 10,000 counts, for example. At such apoint, a second adjusted programming voltage signal can be applied toprogram memory cells in the memory block.

As an example, the second adjusted programming signal can, for example,contain incrementally increasing voltage pulses of 14.9V, 15.4V, 15.9V,and 16.4V, which are lower than the voltage pulses in the programmingvoltage signal depicted in FIG. 2B. Such multiple adjustment embodimentscan allow for smaller adjustments and more flexibility in adjustingdifferent cells in a MLC array, among other benefits.

As mentioned above, decreasing the voltage pulses and/or step upvoltages after a number of processing cycles (e.g., program/erasecycles) can increase program performance by reducing the occurrence ofover-programming which may be caused by electron trapping in the tunneloxide layer of memory cells, for example.

FIG. 3 illustrates a periodic decrease in the Vpgm voltage as the numberof cycles increases for an embodiment of the present disclosure. Theembodiment illustrated in FIG. 3 illustrates the starting programvoltage (Vpgm START) as a function of the number of processing cycles(e.g., program/erase cycles) or hot count.

As used herein, Vpgm START can refer to a single voltage used forprogramming or to the voltage of an initial programming voltage pulse ina series of programming voltage pulses used to program memory cells in amemory block. For instance, Vpgm START for the embodiment in FIG. 2A is16.0V and for the embodiment in FIG. 2B is 15.4V.

The embodiment illustrated in FIG. 3 depicts Vpgm START as beingdecreased multiple times, first, by an amount 310 after 100 cycles and,second, being further decreased by an amount 320 after 1000 cycles. Asstated herein, the decrease can be any suitable amount. For example, theamounts 310 and 320 are on the order of about 0.5V and, in variousembodiments, may or may not be equal amounts.

For example, the value of amounts 310 and 320 may depend on the numberof cycles that have occurred therebetween. For instance, the decreaseamount 320 may be greater or less than the decrease amount 310 dependingon whether the Vpgm START decrease occurs after 100 cycles (as shown) orafter 1,000 cycles.

It is noted that although the example embodiments illustrated in FIGS.2A-2B and 3 depict programming voltage changes at threshold values of100, 1,000, or 10,000 cycles, embodiments are not so limited. That is,embodiments of the present disclosure are not limited to adjusting aprogramming voltage after predetermined threshold hot count values(e.g., processing cycles).

FIG. 4A illustrates a distribution of voltage thresholds (Vt) of a groupof non-volatile memory cells that have been individually programmed intoone of four states before a number of cycles has occurred according toan embodiment of the present disclosure. That is, the embodiment of FIG.4A illustrates a Vt distribution of a group of cells prior to a numberof program/erase cycles, e.g., 100 cycles, 1,000 cycles, etc.

Although there are many forms of memory cells having various numbers ofstates per memory cell, an array with four states per memory cell (e.g.,a storage element) has been chosen for illustrative purposes. In sucharrays, two bits of data can be stored in each memory cell.

In the embodiment represented by the illustration of FIG. 4A, theprogrammed storage elements form memory cell transistors with thresholdlevels that fall into one of threshold distributions 410-1, 430-1,450-1, or 470-1. The distribution 410-1 represents the erased state, orerase level, and is also one of the programmed states (e.g., a “11” inFIGS. 4A and 4B). The distribution 410-1 includes cells having anegative threshold voltage Vt.

The distribution 430-1, including positive threshold voltages,represents data bits “01”. Similarly, the distribution 450-1 represents“00” and the distribution 470-1 represents “10”. The distributions430-1, 450-1, and 470-1 can be referred to as a number of programlevels. An additional number of states, and thus more bits, may beprogrammed into each storage element (e.g., embodiments are not limitedto a system with one erase level and three program levels).

The individual cells are programmed by a series of pulses such asillustrated in FIGS. 2A and 2B. After a block has been erased, all ofits memory cell storage transistors have threshold voltages within thedistribution 410-1.

Upon programming either user data or block overhead data into a numberof memory cells forming all or a portion of a block, programming voltagepulses are applied to those cells whose state is to be changed from “11”to something else. For those transistors to be programmed into the firststate “01” out of erase, the pulsing is terminated when their Vt becomeequal to or greater than the verify level VR01, within the distribution430-1. The states of the cells are verified in between the programmingpulses.

Similarly, pulsing is terminated for those storage transistors to beprogrammed into the “00” state when their Vt become equal to or greaterthan the verify level VR00, within the distribution 450-1. Finally, forthose storage element transistors being programmed into the “10” state,the program pulses are terminated when their Vt reaches their verifylevel VR10, within the distribution 470-1. At that point, the parallelprogramming of the group of the memory cells has been completed.

The individual program verify levels VR01, VR00, and VR10 are coincidentwith the lower extremes of their respective distributions 430-1, 450-1and 470-1. The beginning voltage of the programming pulses of FIGS. 2Aand 2B may be around 16 volts, as an example, and the increment betweenpulses (ΔVpgm or step up voltage) about 0.2-0.6 volts, for example. Thespread of the individual distributions 410-1, 430-1, 450-1 and 470-1 maybe approximately equal to ΔVpgm.

FIG. 4A also illustrates the voltages used to read data from individualcells by determining which of the four threshold states the cell hasbeen programmed. The read voltage levels RD01, RD00, and RD10 arereference voltages used to read the “01”, “00” and “10” storage states,respectively. These voltages can be positioned roughly halfway betweenadjacent ones of the distributions 410-1, 430-1, 450-1 and 470-1. As anexample, RD01 may be about 0.1 V, RD00 may be about 1.0V, and RD10 maybe about 1.9V. Also, the program verify voltages VR01, VR00, and VR10corresponding to the read voltages RD01, RD00, and RD10, respectively,may have respective voltages of about 0.2V, 1.2, and 2.2V.

These are the threshold voltages with which the threshold voltage stateof each memory cell transistor being read is compared. This can beaccomplished by comparing a current or voltage measured from the cellwith reference currents or voltages, respectively.

As discussed further in connection with FIG. 5, FIG. 4A also depictsdifferences, e.g., 415, 435, and 455, between program verify levels andcorresponding read levels for the states. As an example, the difference415, e.g., VR01−RD01, can be on the order of about 0.1V-0.2V. Thedifference between a program verify level and a corresponding read levelmay be referred to herein as a read margin.

Also, as discussed in connection with FIG. 5, the difference between averify level and a corresponding read level can be larger for higherstates, e.g., the difference 435 for the “00” state may be about 0.15Vand the difference 455 for the higher “10” state may be 0.2V.Embodiments of the present disclosure are not limited to these examples.

As the quantity of program/erase cycles, e.g., the “hot count,”increases, data retention can be degraded due to Vt shifts. This dataretention degradation can be higher in MLCs than in single level cells(SLCs) due to the need to distinguish between the multiple states withinVt ranges.

As discussed below in connection with FIGS. 4B and 5, in variousembodiments of the present disclosure, data retention can be increasedby increasing the difference between program verify voltages andcorresponding read voltages for states as the number of process cycles(e.g., program/erase cycles) increases. In various embodiments, both theread voltages and the corresponding verify voltages can be adjusted,e.g., increased, such that the difference between the verify and readvoltage increases as the hot count increases. As discussed in FIG. 6B,in some embodiments, the read voltage may not be adjusted as the hotcount increases while the verify voltage is increased as the hot countincreases. In various embodiments, the difference (read margin) isincreased after a predetermined number of cycles, e.g., 100, 1000,10,000, etc. In various embodiments, the read margin is increased aftermore than one number of cycles. For instance, in some embodiments, theread margin is increased after every 1,000 cycles.

FIG. 4B illustrates a distribution of voltage thresholds (Vt) of a groupof non-volatile memory cells that have been individually programmed intoone of four states after a number of cycles has occurred according to anembodiment of the present disclosure. According to various embodimentsof the present disclosure, FIG. 4B represents the distribution shown inFIG. 4A after a number of processing cycles, e.g., after 100, 100, or10,000 cycles. As one of ordinary skill in the art will appreciate, thevoltage threshold distributions associated with MLCs can shift as thequantity of processing cycles performed on a memory block increases. Anarray with four states per storage element has been chosen forillustrative purposes.

The embodiment shown in FIG. 4B illustrates MLCs having threshold levelsthat fall into one of threshold distributions 410-2, 430-2, 450-2 or470-2. The distribution 410-2 represents the erased state and is alsoone of the programmed states, “11” in this example.

The distribution 410-2 includes cells having a negative thresholdvoltage Vt. The distribution 430-2, including positive thresholdvoltages, represents data bits “01”.

Similarly, the distribution 450-2 represents “00” and the distribution470-2 represents “10”. An additional number of states, and thus morebits, may be programmed into each storage element, i.e., embodiments ofthe present disclosure are not limited to a four state system. Similarto the distribution of FIG. 4A, the distribution of FIG. 4B includesread voltage levels RD01 a, RD00 a, and RD10 a corresponding to the“01,” “00,” and “10” states, respectively. FIG. 4B also includes thecorresponding verify levels VR01 a, VR00 a, and VR10 a. FIG. 4B alsodepicts differences (read margins), e.g., 425, 445, and 465, betweenprogram verify levels and corresponding read levels for the states.

As discussed further below, according to various embodiments of thepresent disclosure, the differences 425, 445, and 465 shown in FIG. 4Bare greater than the differences 415, 435, and 455 shown in FIG. 4A.That is, an initial voltage difference, e.g., difference 415 (beforecycling), can be adjusted to an increased difference, e.g., difference425 (after cycling) in order to improve data retention in a memoryblock.

Also as shown in the embodiment of FIGS. 4A and 4B and discussed furtherbelow, the read voltages and verify voltages after cycling (e.g., RD01a, RD00 a, RD10 a, VR01 a, VR00 a, and VR 10 a) can be increased suchthat they are higher than the corresponding voltage levels (e.g., RD01,RD00, RD10, VR01, VR00, and VR10) prior to cycling. For example, asshown in FIGS. 4A and 4B, RD00 a is greater than RD00 and RD10 a isgreater than RD10.

As discussed herein, various method embodiments of the presentdisclosure include counting a number of process cycles performed on afirst memory block in a memory device. Various method embodimentsinclude adjusting a difference between a program verify voltage and aread voltage associated with the number of process cycles, from aninitial voltage difference, in response to the counted number of processcycles.

In various embodiments, adjusting the voltage difference between theprogram verify voltage and the read voltage includes increasing thevoltage difference. The voltage difference between the program verifyvoltage and the read voltage can be increased from an initial value byabout 100 mV. In various embodiments, the initial voltage difference isin the range of 100-200 mV. In various embodiments, the voltagedifference is increased periodically when the number of process cyclesperformed on the memory block increases past one or more process cyclecount thresholds.

In various embodiments, the initial read voltage, e.g., the read voltageprior to a number of process cycles, is increased based upon thequantity of process cycles. For example, RD00 may be increased from aninitial value of about 0.9V to about 1.0V after 1,000 cycles haveoccurred.

FIG. 5 illustrates verify and read voltage difference changes for anumber of memory cells before and after cycling for an embodiment of thepresent disclosure. The embodiment illustrated in FIG. 5 shows thevoltage difference between a program verify voltage and a read voltageassociated with the three program states (01, 00, and 10) for a fourstate MLC as discussed above. Embodiments of the present disclosure areapplicable to single level cell (SLC) memory arrays as well.

As illustrated in FIG. 5, the difference between a program verifyvoltage level and a read voltage level can be adjusted for a given stateafter a number of program/erase cycles. For instance, in the embodimentof FIG. 5, the initial 100 mV difference (VR01−RD01) is increased to a150 mV difference after 1,000 (1K) cycles. Similarly, the initial 150 mVdifference (VR00−RD00) is increased to a 200 mV difference after 1,000cycles, and the initial 200 mV difference (VR10−RD10) is increased to a300 mV difference after 1,000 cycles.

Also as illustrated in the embodiment of FIG. 5, the initial voltagedifference between a verify voltage and a read voltage, e.g., thedifference before a number of program/erase cycles have been performedon the memory array, can be greater for higher states, or higher programlevels. That is, as shown in FIG. 5, the lowest level (01) has aninitial difference of 100 mV, while the higher levels (00, and 10) haveinitial differences of 150 mV and 200 mV, respectively.

After cycling, e.g., after a number of program/erase cycles have beenperformed on the memory block, the voltage differences associated withthe program levels may be adjusted by different voltage amounts. Invarious embodiments, the voltage difference associated with a higherprogram level is increased by a greater amount than the voltagedifference associated with a lower state. For example, as shown in theembodiment of FIG. 5, after 1,000 cycles, the voltage differenceassociated with the “00” level increases by 50 mV (from 150 mV to 200mV) while the voltage difference associated with the “10” levelincreases by 100 mV (from 200 mV to 300 mV).

As shown in FIG. 5, in various embodiments, the voltage differencebetween a program verify level and a read level can be adjusted based onthe quantity of cycle counts. The voltage difference may be adjusted anumber of times. For example, as shown in FIG. 5, the voltagedifferences for all three program levels depicted are increased when thequantity of cycle counts reaches 1,000 and again when the quantity ofcycle counts reaches about 5,000, in this example. In variousembodiments, the read voltage and/or the verify voltage associated witha state can be adjusted, e.g., increased, based on the quantity of cyclecounts.

It is also noted that in some embodiments, the voltage differencechanges before and after cycling may occur after differing numbers ofcycle counts for different states. For instance, VR01−RD01 may beincreased when 100 processing cycles have occurred, while VR00−RD00 maybe increased when 1,000 processing cycles have occurred and VR10−RD10may by increased when 500 cycles have occurred. Embodiments of thepresent disclosure are not limited to these examples.

FIGS. 6A and 6B illustrate verify and read voltages for a number ofmemory cells as the number of cycles increases according to variousembodiments of the present disclosure. The embodiments of FIGS. 6A and6B illustrate the read voltages and program verify voltages for threeprogram states (01, 00, and 10) of a four state MLC.

The embodiment illustrated in FIG. 6A shows various voltage differencesthat can be adjusted according to various embodiments of the presentdisclosure. In various embodiments, one or both of a read voltage andverify voltage corresponding to a given state can be increased based onthe number of program/erase cycles. For example, in this embodiment, theread voltages RD01, RD00, and RD10 and the verify voltages VR01, VR00,and VR10 are each increased after the cycle count reaches 1,000 andagain when the cycle count reaches 10,000.

The embodiment illustrated in FIG. 6A shows the voltage differencebetween the verify and read voltages for the three program states priorto the cycle count reaching 1,000 counts, after reaching 1,000 counts,and after reaching 10,000 counts. As shown, voltage differences 602,612, and 622 represent the verify/read voltage differences prior to thehot count reaching 1,000 for the 01, 00, and 10 states, respectively.Similarly, voltage differences 604, 614, and 624 represent theverify/read voltage differences after the hot count reaches 1,000 forthe 01, 00, and 10 states, respectively. The voltage differences 606,616, and 626 represent the verify/read voltage differences after the hotcount reaches 10,000 counts for the 01, 00, and 10 states, respectively.

In various embodiments, the verify/read voltage differences for eachstate are increased as the hot count increases. As an example, for the00 program state, the difference 612 can be 0.2V, the difference 614 canbe 0.4V, and the difference 616 can be 0.5V. As previously mentioned, invarious embodiments, the verify/read voltage differences are made largerfor higher program states. For instance, in this example, for the 10state, the difference 622 can be 0.4V, the difference 624 can be 0.5V,and the difference 626 can be 0.6V.

In is noted that FIG. 6A is not drawn to scale. That is, although itappears that the read voltages and verify voltages for the variousstates are increasing by equal amounts, embodiments are not so limited.For example, RD00 can be increased from 0.9V to 1.0V after 1,000 cyclesand from 1.0V to 1.15V after 10,000.

According to the embodiment illustrated in FIG. 6B, the read voltageassociated with each program state of the MLC (01, 00, and 10) does notchange as the cycle count increases. However, in various embodiments,the program verify voltage associated with the program states can beadjusted, e.g., increased, at particular cycle count thresholds, therebyincreasing the difference between the verify level and the read level.For instance, as shown in FIG. 6B, the program verify levels areincreased when the cycle count reaches 1,000 counts and again when thecycle count reaches 10,000 counts.

The amount of the program verify level increase when the count reaches1,000 (A1, B1, and C1) for each program state (01, 00, and 10),respectively, may or may not be equal. Similarly, the amount of theprogram verify level increase when the count reaches 10,000 (A2, B2, andC2) for each program state (01, 00, and 10), respectively, may or maynot be equal to each other and/or may or may not be equal to the amountof the prior increase (A1, B1, and C1).

In some embodiments, the read voltage for one or more program state 01,00, and 10, may remain constant as the cycle count increases while theread voltage for the other states increases as the cycle countincreases. For example, in some embodiments, RD01 may remain constant asthe cycle count increases, while RD00, and RD10 are increased as thecycle count increases.

FIG. 7 is a simplified block diagram of an electronic system 700,according to an embodiment of the present disclosure. Electronic system700 includes a non-volatile memory device 702 that includes an array 704of non-volatile memory cells, an address decoder 706, row accesscircuitry 708, column access circuitry 710, control circuitry 712,Input/Output (I/O) circuitry 714, and an address buffer 716.

The array 704 of non-volatile memory cells has a NAND architecture inaccordance with an embodiment of the disclosure. The memory cells (notshown in FIG. 7) of the array 704 of non-volatile memory cells may befloating-gate memory cells, NROM cells or other type of one-transistornon-volatile memory cells.

Electronic system 700 includes an external processor 720, e.g., a memorycontroller or host processor, electrically connected to memory device702 for memory accessing. The memory device 702 receives control signalsfrom the processor 720 over a control link 722. The memory cells areused to store data that are accessed via a data (DQ) link 724.

Address signals are received via an address link 726 that are decoded ataddress decoder 706 to access the memory array 704. Address buffercircuit 716 latches the address signals. The memory cells are accessedin response to the control signals and the address signals.

The control link 722, data link 724 and address link 726 can becollectively referred to as access lines. It will be appreciated bythose skilled in the art that additional circuitry and control signalscan be provided, and that the memory device detail of FIG. 7 has beenreduced to facilitate ease of illustration.

FIG. 8 is an illustration of an exemplary memory module 800. Memorymodule 800 is illustrated as a memory card, although the conceptsdiscussed with reference to memory module 800 are applicable to othertypes of removable or portable memory, e.g., USB flash drives, and areintended to be within the scope of “memory module” as used herein. Inaddition, although one example form factor is depicted in FIG. 8, theseconcepts are applicable to other form factors as well.

In some embodiments, memory module 800 will include a housing 805 (asdepicted) to enclose one or more memory devices 810, though such ahousing is not essential to all devices or device applications. At leastone memory device 810 is a non-volatile memory having NAND architecturein accordance with an embodiment of the present disclosure.

Where present, the housing 805 includes one or more contacts 815 forcommunication with a host device. Examples of host devices includedigital cameras, digital recording and playback devices, PDAs, personalcomputers, memory card readers, interface hubs and the like.

For some embodiments, the contacts 815 are in the form of a standardizedinterface. For example, with a USB flash drive, the contacts 815 mightbe in the form of a USB Type-A male connector.

For some embodiments, the contacts 815 are in the form of asemi-proprietary interface, such as might be found on CompactFlash™memory cards licensed by SanDisk Corporation, Memory Stick™ memory cardslicensed by Sony Corporation, SD Secure Digital™ memory cards licensedby Toshiba Corporation and the like. In general, however, contacts 815provide an interface for passing control, address and/or data signalsbetween the memory module 800 and a host having compatible receptors forthe contacts 815.

The memory module 800 may optionally include additional circuitry 820which may be one or more integrated circuits and/or discrete components.For some embodiments, the additional circuitry 820 may include a memorycontroller for controlling access across multiple memory devices 810and/or for providing a translation layer between an external host and amemory device 810. For example, there may not be a one-to-onecorrespondence between the number of contacts 815 and a number of 810connections to the one or more memory devices 810.

Thus, a memory controller could selectively couple an I/O connection(not shown in FIG. 8) of a memory device 810 to receive the appropriatesignal at the appropriate I/O connection at the appropriate time or toprovide the appropriate signal at the appropriate contact 815 at theappropriate time. Similarly, the communication protocol between a hostand the memory module 800 may be different than what is required foraccess of a memory device 810.

A memory controller could then translate the command sequences receivedfrom a host into the appropriate command sequences to achieve thedesired access to the memory device 810. Such translation may furtherinclude changes in signal voltage levels in addition to commandsequences.

The additional circuitry 820 may further include functionality unrelatedto control of a memory device 810 such as logic functions as might beperformed by an application specific integrated circuit (ASIC). Also,the additional circuitry 820 may include circuitry to restrict read orwrite access to the memory module 800, such as password protection,biometrics or the like. The additional circuitry 820 may includecircuitry to indicate a status of the memory module 800.

For example, the additional circuitry 820 may include functionality todetermine whether power is being supplied to the memory module 800 andwhether the memory module 800 is currently being accessed, and todisplay an indication of its status, such as a solid light while poweredand a flashing light while being accessed. The additional circuitry 820may further include passive devices, such as decoupling capacitors tohelp regulate power requirements within the memory module 800.

CONCLUSION

Methods, devices, systems, and modules embodiments for memory cyclevoltage adjustment have been described. Adjusting a program voltage froman initial voltage to an adjusted voltage as a counted number ofprocessing cycles increases can maintain programming reliability asprogramming speed increases. Also, adjusting a difference between aprogram verify voltage and a read voltage as the counted number ofprocessing cycles increases can improve data retention while maintainingfast programming speeds.

Although specific embodiments have been illustrated and describedherein, those of ordinary skill in the art will appreciate that anarrangement calculated to achieve the same results can be substitutedfor the specific embodiments shown. This disclosure is intended to coveradaptations or variations of various embodiments of the presentdisclosure.

It is to be understood that the above description has been made in anillustrative fashion, and not a restrictive one. Combination of theabove embodiments, and other embodiments not specifically describedherein will be apparent to those of skill in the art upon reviewing theabove description.

The scope of the various embodiments of the present disclosure includesother applications in which the above structures and methods are used.Therefore, the scope of various embodiments of the present disclosureshould be determined with reference to the appended claims, along withthe full range of equivalents to which such claims are entitled.

In the foregoing Detailed Description, various features are groupedtogether in a single embodiment for the purpose of streamlining thedisclosure. This method of disclosure is not to be interpreted asreflecting an intention that the disclosed embodiments of the presentdisclosure have to use more features than are expressly recited in eachclaim.

Rather, as the following claims reflect, inventive subject matter liesin less than all features of a single disclosed embodiment. Thus, thefollowing claims are hereby incorporated into the Detailed Description,with each claim standing on its own as a separate embodiment.

1. A method of programming a memory cell with a programming signal, theprogramming signal comprises a plurality of programming pulses, whereinthe plurality of programming pulses incrementally change by at least onestep voltage between successive pulses of the plurality of programmingpulses, the method comprising: determining a quantity of processingcycles performed on a group of memory cells; and adjusting the at leastone step voltage between successive pulses of the plurality ofprogramming pulses responsive to the determined quantity of processingcycles.
 2. The method of claim 1, wherein determining a quantitycomprises counting a number.
 3. The method of claim 1, wherein the atleast one step voltage comprises a plurality of different step voltages.4. The method of claim 1, wherein the at least one step voltagecomprises a step up voltage.
 5. The method of claim 1, wherein the groupof cells comprises a block of memory cells.
 6. The method of claim 1,further comprising decreasing an initial programming voltage associatedwith the programming signal responsive to the determined quantity ofprocessing cycles.
 7. A method comprising; determining a quantity ofprocessing cycles performed on a group of memory cells; and adjusting aninitial programming pulse of a programming signal comprising a pluralityof programming pulses from an initial program voltage to an adjustedvoltage, in response to the determined number of processing cycles,wherein the plurality of programming pulses comprise a number of steppedvoltage pulses having at least two different step voltages betweensuccessive pulses of the plurality of programming pulses.
 8. The methodof claim 7, wherein determining the quantity includes counting a number.9. The method of claim 7, wherein the at least two different stepvoltages comprise a plurality of different step up voltages.
 10. Themethod of claim 7, wherein the group of cells comprises a block ofmemory cells.
 11. The method of claim 7, wherein adjusting the initialprogramming pulse comprises decreasing the initial program voltage tothe adjusted voltage.
 12. The method of claim 7, wherein the methodincludes adjusting the number of stepped voltage pulses periodically asthe determined number of processing cycles increases past a number ofprocessing cycle count thresholds.
 13. A memory device comprising; anarray of memory cells; and control circuitry coupled to the array andconfigured to perform a method of programming a memory cell with aprogramming signal, the programming signal comprises a plurality ofprogramming pulses, wherein the programming pulses incrementally changeby at least one step voltage, the method comprising: determining aquantity of processing cycles performed on a group of memory cells; andadjusting the at least one step voltage between successive pulses of theplurality of programming pulses responsive to the determined quantity ofprocessing cycles.
 14. The memory device of claim 13, whereindetermining a quantity comprises counting a number.
 15. The memorydevice of claim 13, wherein the at least one step voltage comprises aplurality of different step voltages.
 16. The memory device of claim 13,wherein the at least one step voltage comprises a step up voltage. 17.The memory device of claim 13, wherein the group of memory cellscomprises a block of memory cells.
 18. The memory device of claim 13,wherein adjusting the at least one step voltage includes decreasing aninitial programming voltage associated with the programming signal. 19.A memory device comprising; an array of memory cells; and controlcircuitry coupled to the array and configured to: determine a quantityof processing cycles performed on a group of memory cells; and adjust aninitial programming pulse of a programming signal comprising a pluralityof programming pulses from an initial program voltage to an adjustedvoltage, in response to the determined number of processing cycles,wherein the plurality of programming pulses comprise a number of steppedvoltage pulses having at least two different step voltages betweensuccessive pulses of the plurality of programming pulses.
 20. The deviceof claim 19, wherein the at least two different step voltages comprise aplurality of different step up voltages.
 21. The device of claim 19,wherein adjusting the initial programming pulse includes decreasing theinitial program voltage to the adjusted voltage.
 22. The device of claim19, wherein the control circuitry is configured adjust the number ofstepped voltage pulses periodically as the determined number ofprocessing cycles increases past a number of processing cycle countthresholds.